Mercedes-AMG A45 S W177

The AMG Speedshift 8G-DCT (724.1) is the most frequently reported problem area on the W177 A45 S. In cold weather (below approximately 10 degrees Celsius), selecting reverse from standstill can produce a metallic crunching sound. Any metallic crunch (not a normal clunk) indicates a significant internal issue, typically worn synchro rings or output shaft damage. Some owners report having the problem on two consecutive cars, suggesting it is a design-related weakness rather than a rare defect. Mercedes may address it under warranty with new output shafts or a complete gearbox replacement. Out of warranty, mechatronic unit repair costs approximately 2,500-4,000 euros, while complete gearbox replacement runs 6,000-8,000 euros. Software updates can improve shift quality but do not resolve mechanical wear. Regular gearbox oil changes every 60,000 km are strongly recommended despite Mercedes listing the fluid as a lifetime fill.

The M139 engine's water pump is positioned beneath the exhaust manifold and turbocharger, making it one of the most difficult water pump replacements on any modern car. The repair requires removal of the turbocharger, exhaust manifold, and disconnection of numerous oil and coolant lines, totaling 8-12 hours of labor. Parts cost approximately 400-600 euros, but labor adds 1,000-2,000 euros at a specialist or 2,000-3,000 euros at a Mercedes dealer. Failure typically occurs between 70,000-100,000 km. Symptoms include coolant warning lights, visible coolant leaks, sweet smell from the engine bay, or poor cabin heating. The thermostat housing is often replaced at the same time as a preventive measure, adding approximately 200-300 euros for the part.

Mercedes issued a Technical Service Bulletin (NHTSA MC-10225806) for broken valve springs in the M139 engine. Symptoms include check engine light, rough running, difficulty starting, and problems with the ECO start/stop function. Diagnosis requires removing the cylinder head cover and inspecting the valve springs directly. While this is a documented issue, it appears to be relatively rare compared to the gearbox problems. A compression test or running compression test can identify the issue before disassembly. Repair involves replacing the affected valve springs and potentially the cylinder head if damage has occurred. When caught early, spring replacement alone costs approximately 2,000-2,500 euros. If valve or piston damage has occurred, costs escalate to 3,500-4,500 euros.

The A45 S uses a fully variable rear axle with two multi-plate clutches for torque vectoring. This system is calibrated to send more power rearward than on the standard A35, which means the clutch packs slip more frequently and generate more heat. The clutch oil degrades faster than on less aggressive setups. Symptoms start as a light whining noise from the rear, typically noticed between 40,000-80,000 km if oil has not been changed regularly. Mercedes recommends rear differential oil changes at every service (approximately every 15,000-20,000 km). If maintained, the system is reliable. However, if the oil has been neglected, clutch pack wear accelerates and a full rebuild can cost 2,000-3,000 euros. The center section gear set uses a separate hypoid oil that Mercedes considers a lifetime fill.

Interior rattles are extremely common on the W177 platform and affect the A45 S just as much as lesser models. The most frequently reported sources are the MBUX infotainment screen (where screws clamp plastic to floating metal clips allowing movement), the three aluminum-colored air vent rings that vibrate against their housings, the instrument cluster surround, door cards, and seats. Cold weather and rough roads exacerbate the problem. Many owners report multiple dealer visits (3 or more) with limited success in permanent fixes. Dealers can apply insulating material behind the screen unit per Mercedes guidelines, but this does not always resolve the issue. DIY fixes using silicone spray on shoulder mounts and clips can help temporarily. While not a safety or reliability concern, the rattles are a significant quality-of-life issue on a car costing 60,000+ euros new.

Mercedes has acknowledged a defect with Mars Red paint across multiple model ranges, resulting in a class action settlement providing extended warranty coverage (up to 15 years/150,000 miles). Jupiter Red also shows similar issues but is not officially covered by the settlement. The defect manifests as paint lacquer bubbling that spreads across doors, A-pillars, roof, bonnet, and spoiler. Bubbles eventually burst, causing the clear coat to peel off. This affects cars regardless of how they are cared for, as it is a manufacturing and formulation defect. Full respray of affected panels costs 2,000-5,000 euros depending on the extent. Other paint colors (white, grey, black) do not appear to be affected. If purchasing an A45 S in Jupiter Red or Mars Red, inspect all panels carefully and budget accordingly.

The W177 uses a small auxiliary battery in the boot that powers the MBUX infotainment system and various electronic modules. When this battery degrades, it can cause the MBUX screen to go completely black, freeze, or behave erratically. The MBUX system is highly dependent on stable voltage from this secondary battery. Replacement of the auxiliary battery itself costs only 100-200 euros. However, if the voltage instability has corrupted MBUX software or damaged the LVDS data cables between modules, more extensive repairs are needed. A software reflash may resolve it for free, but hardware failures require screen replacement (1,500+ euros) or control unit replacement. Cars that sit for extended periods without driving are most susceptible. Using the MBUX sleep mode function when parking for extended periods can help manage drain.